The purpose of this investigation is to study the regulation of intracellular protein turnover in Escherichia coli and to isolate and characterize an in vitro cell-free system which catalyzes the degradation of glutamine synthetase (GS). The current studies indicate that degradation of GS by E. coli extracts might be a two-step process consisting of inactivation followed by proteolysis. The inactivation reaction was first observed in extracts of nitrogen starved Klebsiella aerogenes (Fulks, R.M. (1977) Fed. Proc. 36, 3420). Similar inactivation occurs when GS is incubated with ascorbate (Levine, R.L. (1980) Fed Proc. 39,401) and with catalase-free E. coli extracts. This reaction requires Fe 3 ion, NADPH, O2 and is inhibited by catalase. Rabbit liver microsomal P450 mixed function oxidase system consisting of cytochrome P450 and NADPH cytochrome C reductase catalyzes a similar inactivation and this system has been used as a model to study inactivation without proteolysis. Techniques used in these studies have included polyacrylamide pore gradient electrophoresis, isotopic labeling, chromatographic techniques, autoradiography, enzymatic assay of functional proteins, high pressure liquid chromatography.